1. Field of the Invention
This invention relates generally to equalizers employed in data and clock recovery in telecommunication systems, more particularly to delay line implementations employed in a feed-forward equalizer (FFE) or a decision feedback equalizer (DFE), and most particularly to an active delay line employed in such equalizers.
2. Description of the Related Art
The demand for bandwidth in optical telecommunication networks has pushed operating network transmission rates for long-haul backbone to 10 Gb/s, which is more susceptible to chromatic dispersion and polarization mode dispersion. This will become more of a problem with the advent of backbones operating at transmission rates of 40 Gb/s. Optical domain dispersion compensation modules (DCMs) have been traditionally employed to restore the transported signals at the optical receiver to be sufficiently discemable at a CDR circuit and make highly competent decisions as to binary 1's and 0's in the signal data stream. The primary source of signal dispersion is signal pulse broadening due to fiber dispersion on the transported signal. In general, these DCMs include optical dispersion compensating fiber (DCF) which is capable of providing dispersion compensation for multiple signals received at the receiver as a WDM or DWDM signal. However, in certain cases, such as when only a few signal channels are in operation, such as due to less than all channels in a system concurrently operating for a period of time due to initial stages of network deployment so that the expensive nature of DCMs will raise network costs, particularly at the time of initial network installation.
Instead of deploying optical domain solutions, another traditional approach is the electrical domain solution employing electronic dispersion compensation circuits which are also referred to as adaptive equalizers. They are of particular interest because they potentially offer lower cost and greater flexibility in system design as well as a way to mitigate fiber impairments imposed on the transport WDM/DWDM signal without any required foreknowledge of the optical link or span characteristics and parameters.
As known in the art of equalizers, there are two different types: the feedforward equalizer (FFE) and the decision feedback equalizer (DFE). At network transmission rates at 10 GB/s, FFE is typically implemented as a delay line plus a buffer or a transversal filter since they can compensate for many types of linear distortion. These type of filters may be as long as 8.4 mm on an equalizer chip. Such equalizers have a delay line comprising a series of connected passive delays where taps are provided between the delays and the taped delayed outputs are multiplied by equalization coefficients or weights and are then summed to generate an equalized output. Other delay line architectures have been proposed such as the adaptive distributed circuit type comprising two transmission lines with intermediate gain stages which amplify the output signals from the first transmission line by a gain proportional to a corresponding equalization coefficient or weight. A problem with these types of equalizers is that the delay lines are comparatively large, taking a lot of IC chip real estate.
What is desired is an adaptive equalizer that is more cost effective by being more compact as implemented in MMIC.